The present invention relates to communications apparatus and methods, and more particularly, to spread spectrum communications apparatus and methods.
Wireless communications systems are widely used to communicate voice and other data, and the use of such systems is increasing through the development of new applications. For example, in addition to traditional voice telephony applications, wireless systems are increasingly being used to provide data communications services such as internet access and multimedia applications.
FIG. 1 illustrates a typical direct sequence spread spectrum (DS-SS) signal generator, as might be used in a code division multiple access (CDMA) communications system. A data sequence is spread by a spreading sequence, which typically has a much higher baud rate. The spread signal thus produced is passed through a pulse shaping filter to generate a baseband signal s(t), which is given by:                                           s            ⁡                          (              t              )                                =                                    ∑                              i                =                                  -                  ∞                                            ∞                        ⁢                                          α                ⁡                                  (                  i                  )                                            ⁢                                                f                  i                                ⁡                                  (                                      t                    -                    iT                                    )                                                                    ,                            (        1        )                                                                    f              i                        ⁡                          (              t              )                                =                                    ∑                              l                =                0                                            N                -                1                                      ⁢                                                            a                  i                                ⁡                                  (                  l                  )                                            ⁢                              p                ⁡                                  (                                      t                    -                                          lT                      c                                                        )                                                                    ,                            (        2        )            where fi(t) is the spreading waveform for the ith symbol, α(i) is the ith data symbol, ai(l) is the lth “chip” of the spreading sequence in the ith symbol interval, N is the processing gain, Tc is the chip duration, T=NTc is the symbol duration, and p(t) is the chip pulse. The baseband signal s(t) is then typically modulated by a carrier signal, and the resultant data-modulated carrier signal is transmitted in a communications medium, e.g., in air, wireline or other medium.
The channel experienced by a transmitted wireless DS-SS signal is typically modeled as a dispersive channel with an impulse response of the form:                               g          ⁡                      (            t            )                          =                              ∑                          l              =              0                                      L              -              1                                ⁢                                    g              l                        ⁢                          δ              ⁡                              (                                  t                  -                                      τ                    l                                                  )                                                                        (        3        )            where L is the number of multipaths, and gl and τl are the complex-valued attenuation factor and delay for the lth path, respectively. The baseband equivalent signal received over such a channel can be expressed as:                                           y            ⁡                          (              t              )                                =                                                    ∑                i                            ⁢                                                α                  ⁡                                      (                    i                    )                                                  ⁢                                                      h                    i                                    ⁡                                      (                                          t                      -                      iT                                        )                                                                        +                          n              ⁡                              (                t                )                                                    ,                                  ⁢                  where          :                                    (        4        )                                                                    h              i                        ⁡                          (              t              )                                =                                    ∑                              l                =                0                                            L                -                1                                      ⁢                                          g                l                            ⁢                                                f                  i                                ⁡                                  (                                      t                    -                                          τ                      l                                                        )                                                                    ,                            (        5        )            and n(t) includes thermal noise and multi-user interference.
Conventionally, a RAKE receiver 200 as shown in FIG. 2 may be used to recover information from a DS-SS signal. A radio processor 220 converts a received signal received via an antenna 210 to baseband, including filtering the signal based on the chip pulse shape and sampling the result. A RAKE processor 230 includes a correlator 232 that correlates the sampled signal with a spreading sequence at a plurality of offset correlation times. For example, the correlator may include J RAKE “fingers,” each matched to one signal ray (J=L), and a correlation between the received signal and a delayed version of the spreading sequence may be calculated at each finger. A combiner 234 typically employs maximum ratio combining (MRC) to combine the correlation values produced by the correlator 232, typically based on channel coefficient estimates produced by a channel estimator 240. Channel delay estimates generated by the channel estimator 240 may be used to determine the offset correlation times used by the correlator 232.
One important feature of so-called “third generation” wireless communications systems is the ability to provide services with a wide range of data rates to meet the varying information transmission needs of various services such as voice and data. For example, in IS-2000 and wideband CDMA (W-CDMA) wireless communications systems, multiple data rates may be achieved by using various combinations of codes, carriers and/or spreading factors. More particularly, in W-CDMA systems, the spreading factors of physical channels may range from 256 to 4, providing corresponding data rates from 15K baud per second (bps) and 0.96 Mbps.
For a physical channel employing a low spreading factor, a conventional RAKE receiver may not perform well if the channel is dispersive. This performance degradation may arise because the processing gain provided by signal spreading may not be sufficient to reject inter-symbol interference (ISI) arising from multipath propagation. Consequently, user throughput and coverage may be limited by multipath delay spread.